For years, the cornerstones of cancer treatment have been surgery, chemotherapy, and radiation therapy. Over the last decade, targeted therapies—drugs that target cancer cells by homing in on specific molecular changes seen primarily in those cells—have also emerged as standard treatments for a number of cancers. A newer approach utilizing immunotherapy involves engineering immune cells to recognize and attack tumors.
Normal hematopoietic stem cells (HSCs) are primed to be highly sensitive to retinoids (compounds specific for the Retinoic Acid Receptor or RAR) but are maintained in a retinoid signaling-naïve state by isolating them from physiologic levels of retinoids. The bone marrow microenvironment, by expression of the enzyme CYP26 metabolically inactivates retinoic acid, regulates the exposure of the bone marrow to retinoids. This mechanism (CPY26-mediated retinoid metabolism) is dynamic and used by the bone marrow stroma to match HSC behavior to physiological needs. For example, steady state low levels of retinoids in the bone marrow niche maintains HSCs in a quiescent state, while during situations of stress (i.e., exposure to radiation or chemotherapy) higher retinoid levels are maintained to recruit HSCs into cell division and rescue hematopoiesis.
In subjects with hematologic malignancies, cancer HSCs are protected from retinoids by stromal CYP26, in a similar fashion to the normal situation. However, because of other alterations in the bone marrow niche in hematologic malignancies, such as differences in aldehyde dehydrogenase (ALDH) activity, there exists a therapeutic window for retinoids to be useful in the treatment of hematologic malignancies. Expression of CYP26 by the bone marrow microenvironment contributes to the protection of immature acute myeloid leukemia (AML) cells from all-trans retinoic acid (ATRA) and may explain why ATRA is not effective in treating AML. Exposure to pharmacological concentrations of ATRA, acting through retinoic acid receptor gamma (RARγ), induces CYP26 expression in the bone marrow microenvironment, thus protecting the cancer stem cells therein from retinoid activity. This mechanism also shields non-hematopoietic metastatic tumor cells in the bone marrow. However, the use of retinoid analogs which are not inactivated by CYP26 enables such retinoids to terminally differentiate, and thus eliminate, the cancer HSCs from the protective bone marrow niche. Since such differentiation is mediated by RARα and the use of RARα specific analogs, which are CYP26 resistant, enables the therapeutic differentiation-inducing activity without inactivation by the CYP26 enzyme.
Thus, the combination of RAR agonists, to induce differentiation of cancer stem cells, and targeted immunotherapy can be particularly useful in treating cancer.